Part III. Interaction of Photons with Matter. Photon Absorption Length λ. Intensity Attenuation. Three effects are important: Scintillators

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1 Part III Interaction of Photons with Matter Scintillators Photodetectors Cherenkov detectors Transition radiation detectors Calorimeters - shower development - electromagnetic calorimeters - hadronic calorimeters σ Mb Cross section (barns/atom) kb 0mb Cross section (barns/atom) b b Mb kb b σ p.e. σ coherent σ incoh (a) Carbon ( Z = 6) experimental σ tot Kohlenstoff (leicht) σ p.e. σ coherent κ N σ σ incoh nuc κ e (b) Lead ( Z = 82) experimental σ tot Blei (schwer) minimum κ N κ e C Pb Three effects are important: Photoeffect: γ + atom e + atom + Z σ Ph E 3.5 Comptoneffect: γ + atom γ + e + atom + lne σ Compton Z E Pairproduction: γ + nucl. e + + e - + nucl. σ Pair Z 2 für E γ» 2m e c 2 σ nuc 0 mb 0 ev kev MeV GeV 00 GeV Photon Energy 0eV kev MeV GeV 00GeV E γ carsten.niebuhr@desy.de Particle Detectors 3 carsten.niebuhr@desy.de 2 Particle Detectors 3 Intensity Attenuation Intensity attenuation: I( x) = I 0 exp( µx), N with mass absorption coefficient µ A σ = [cm - ] A strongly energy dependent Photon Absorption Length λ Definition of mass absorption coefficient: [g cm -2 N λ = ] with µ A σ = ( µ ρ) A Z σ lne Ph E 3.5 σ Compton Z σ E Pair Z 2 00 connection between radiation length and high energy limit for pair production: 7 σ E» GeV 9 --4αr 2Z 2 ln e Z A = N A X 0 I( x) = I 0 exp 7 probability for pair production after x X 0 traversing one X 0 is 54%. Absorption length λ (g/ cm 2 ) H C Fe Si Sn Pb ev 00 ev kev 0 kev 00 kev MeV 0 MeV 00 MeV GeV 0 GeV 00 GeV Photon energy E γ carsten.niebuhr@desy.de 3 Particle Detectors 3 carsten.niebuhr@desy.de 4 Particle Detectors 3

2 Scintillation Detectors Phenomenon of scintillation known since long. Extensive use only after invention of photomultiplier in 944. Since then significant development of this technology. Advantage for detection of particles and photons: light guide simplicity and robustness signal speed high density large signals good time and energy measurement Now also scintillating fibres available position resolution as well There are different mechanisms in: anorganic crystals organic substances scintillator photo detector Example: Sodium-Iodide NaI insulator with bandgap of 7eV replace 0.% of sodium atoms with socalled activators: thallium atoms - shift of light energy into visible regime: (advantageous for detection via photo cathode) - enhanced light yield - reduced re-absorption exciton creation by charged particles Inorganic Crystals excitons move in crystal until they reach activator energy release by photon emission (3eV λ 400nm) energy for this wavelength the material is transparent conduction band exciton band activator states 3eV valence band exciton electron 7eV hole electron traps decay time τ 230 ns carsten.niebuhr@desy.de 5 Particle Detectors 3 carsten.niebuhr@desy.de 6 Particle Detectors 3 Mechanism Excited vibrational modes of molecules de-excite by emission of UV light. This UV light is then transformed into visible light by so called wave length shifters that are added to the material. Organic Scintillators Comparison Organic vs Inorganic Scintillators Inorganic Organic Mono crystals - Napthalen (C 0 H 8 ) - Anthrazen (C 0 H 0 ) - p-terphenyl (C 58 H 4 ) Liquid- and plastic- scintillators consist of organic substance (polystyrol) plus scint. molecules ( %) in addition: secondary fluor compounds as wavelength shifters Inorganic crystals well suited for calorimetric applications (high light yield and small radiation length) Plastic scintillators fast particle registration (trigger) carsten.niebuhr@desy.de 7 Particle Detectors 3 carsten.niebuhr@desy.de 8 Particle Detectors 3

3 Light Collection Conversion of Scintillation Light Scintillation light must be converted into electrical signal. Requirement high sensitivity, i.e. high "quantum efficiency": Q.E. = N photoelectrons / N photon Commonly used photo detectors wavelength shifter bars gas based systems - e.g. RICH detectors vacuum based systems - photomultiplier Threshold for photo-emission of various materials plexiglas lightguide total reflection in optical fibres solid state detectors - photodiodes etc. λ (nm) carsten.niebuhr@desy.de 9 Particle Detectors 3 carsten.niebuhr@desy.de 0 Particle Detectors 3 Photomultiplier Avalanche Photo Diode APD γ cathode focussing dynods vacuum vessel (glass) anode large reverse bias voltage of V high internal electric field leads to avalanche formation -U D voltage divider Example: 0 dynodes, each with gain factor N total gain M = g i = i = g = 4 typical gain G carsten.niebuhr@desy.de Particle Detectors 3 carsten.niebuhr@desy.de 2 Particle Detectors 3

4 Hybrid Photo Diode HPD Cherenkov Radiation Combination of: photocathode - like in PMT acceleration region in vacuum - V = 0-20 kv silicon detector e V 20keV 3 - Gain G = = W Si 3.6eV - Poisson statistics with n = 5000 extremly good pulse height resolution. Single photon counting. Backscattering from silicon surface If particle velocity is larger than speed of light in medium: - asymmetric polarisation of medium - emission of Cherenkov light Opening angle of emission cone: ( c n) t cos( θ c ) = = βc t β n l light =(c/n) tl θ l part =βc t wave front finite thickness Atomic Dipoles threshold at β thr = -- (i.e. θ ) n C 0 maximum opening angle: θ max = arccos -- (für ) n β photon yield small (λ nm): dn dx = 500sin 2 θ C [cm ] carsten.niebuhr@desy.de 3 Particle Detectors 3 carsten.niebuhr@desy.de 4 Particle Detectors 3 Emissionswinkel emission angle θ BaO; AgCl Cherenkov Angle vs β Liquids and Solids Gases Plexiglas; BaF 2 n=2.0.5 H 2 O Geschwindigkeit β velocity β θ Isobutan Freon Propan Äthan n=.003 Luft Geschwindigkeit velocity β Photonen/cm ( nm) Flüssigkeiten Liquids and und Solids Feststoffe n=2.0.5 BaO; AgCl Geschwindigkeit velocity β β Photon Yield vs β H 2 O Plexiglas; BaF Photonen/m ( nm) Gases n=.003 Isobutan Freon Äthan Propan Luft Geschwindigkeit velocity β carsten.niebuhr@desy.de 5 Particle Detectors 3 carsten.niebuhr@desy.de 6 Particle Detectors 3

5 Example: Ring Imaging Cherenkov Counter RICH K Transition Radiation Even below the threshold for Cherenkov-radiation photons can be emitted when charged particles cross boundaries between media with different dielectric constants. dipol radiation Radiated energy per boundary: Hermes RICH detector π e W =, i.e. only significant for highly 3 --α h ω 2π p γ γ relativistic particles (e ± ) particle identification θ [rad] π K p X-ray photons are emitted in a forward cone; θ γ transition radiation only occurs very close to the track p [GeV] carsten.niebuhr@desy.de 7 Particle Detectors 3 carsten.niebuhr@desy.de 8 Particle Detectors 3 Transition Radiation Detectors Use of Transition Radiation Detectors at LHC high Z (e.g. Xe Z=54) ATLAS TRT Transition Radiation Tracker ALICE TRD 30 GeV π 30 GeV e TR Radiator Straws C-fiber s 000 foils low Z TR hits characterised by: large amplitude occur preferentially at start of the track Application: distinguish high energetic electrons from pions discrimination possible for momenta from about GeV/c to ~00 GeV/c Tension plate Electrons with radiat Electrons without radiator carsten.niebuhr@desy.de 9 Particle Detectors 3 carsten.niebuhr@desy.de 20 Particle Detectors 3

6 Intermediate Summary Scintillators inorganic crystals (NaI, CsI, BGO, PbWO 4,...) - energy loss by ionisation transfer to luminiscent centers radiate photons - high light yield, small radiation length calorimeters organic (polystyrene, polyvinyltoluene,...) - molecules get excited energy released as optical photons Fluors act as wavelength shifter - lower light yield, but faster signals trigger counters Why do we need Calorimeters? Recall: for tracking in magnetic field we have σ( p T ) p T p T L 2 momentum (energy) measurement degrades linearly with increasing energy size of detector L E only detection of charged particles σ/e hadr. calorimeter tracker Conversion of Scintillation Light photo multiplier solid-state photon detectors (APD, HPD, SiPM,...) Energy loss processes other than ionisation (used for particle identification) Cherenkov radiation - threshold effect β thr = n, if particle moves faster than light in medium; but small light yield Transition radiation - fast particles radiate if they cross boundaries sensitivity to γ ; but small light yield In contrast (as we will see) for calorimeters: σ( E) detection of - photons - neutral hadrons for high energy detectors calorimeters are indispensable components elmag.. calo. E [GeV] carsten.niebuhr@desy.de 2 Particle Detectors 3 carsten.niebuhr@desy.de 22 Particle Detectors 3 Electromagnetic Shower Development Interaction of photons and electrons above 0 MeV dominated by pairproduction γ e + e - Bremsstrahlung e ± e ± γ which are both characterised by X 0. Alternating sequence leads to shower which stops if energy of particles < E c. Simple model for shower development initiated by photon of energy E 0 =E γ : within X 0 γ produces e + e - pair assume symmetric energy sharing E e =E γ /2 e + e - radiate photon after X 0 E γ =E e /2 number of particles at depth t = x X 0 : N() t = 2 t with energy Et () = E 0 2 t multiplication continues until energy falls below critical energy : E c E 0 2 t MAX = Energy Loss of Electrons In addition to energy loss by ionisation high energy particles also loose energy due to interaction with the Coulomb field of the nuclei: Bremsstrahlung Due to their small mass this effect is especially prominent for electrons (positrons): de dx Z 2 E m 2 it is useful to introduce radiation length X 0 x energy attenuation: E = E exp X 0 Example: Cu approx.: critical energy E c : 76g cm X 2 A 0 = Z( Z + ) ln( 287 Z) from then on shower particles are only absorbed. Position of shower maximum: lne 0 E c t MAX = lne ln2 0 de Brems de = collision dx dx 60MeV approximately: E c = Z +.24 E c αlne carsten.niebuhr@desy.de 23 Particle Detectors 3 carsten.niebuhr@desy.de 24 Particle Detectors 3

7 Shower Depth vs Energy Example: µ-induced Shower typical range of depth for EM calorimeter carsten.niebuhr@desy.de 25 Particle Detectors 3 carsten.niebuhr@desy.de 26 Particle Detectors 3 Multiple scattering of the e ± causes a broadening of the shower also in the transverse direction: contribution from electrons with E E c dominates the shower width can be characterized by the so-called 2MeV Moliere-Radius R M = X E 0 c meaning: 90(95)% of shower energy is contained in cylinder with radius R M (2R M ) around the shower axis Shower Containment: transverse: R 95% = 2R M Shower Development dn t α e βt dt - Example lead glass: R M =.8X cm R 95% 7 cm longitudinal: L 95% = t MAX Z [X 0 ] [with t MAX = ln( E 0 E c ) ( ln2) ] - Example: 00 GeV e- in lead glass (E c =.8 MeV t MAX 3, L 95% 23) Stochastic Fluctuations Number of particles at shower maximum increases linearly with initial energy: N MAX = N( t MAX ) = E 0 E c Total number of particles in the shower N tot N MAX = E 0 E c If response of calorimeter is proportional to number of shower particles it acts as a linear device for energy measurements Even for a perfect detector there are intrinsic statistical limitations for the energy resolution: E 0 - total track length T N tot X X E 0 c - detectable track length T det = F( ξ) T with ξ = E cut E c [above energy threshold E cut ] σ( E) σ( T - for relative energy resolution det ) = = E T det T det E carsten.niebuhr@desy.de 27 Particle Detectors 3 carsten.niebuhr@desy.de 28 Particle Detectors 3

8 Energy Resolution In general the energy resolution of a calorimeter can be parametrised as: Calorimeter Types Stochastic Term stochastic fluctuations in shower development sampling fluctuations in case of sampling calorimeter σ( E) a E = b c E Ē -- Constant Term inhomogeneities dead material non-linearities leakage Noise Term electronic noise radioactivity pile-up Homogeneous calorimeters: detector = absorber good energy resolution limited spatial resolution (particularly in longitudinal direction) only used for electromagnetic calorimetry Sampling calorimeters: detectors and absorber are separated only fraction of the energy is sampled heavy absorber material: compact design energy resolution limited by sampling fluctuations good spatial resolution due to segmentation photo-electron statistics inter-calibration between individual cells can be used for electromagnetic and hadronic calorimetry carsten.niebuhr@desy.de 29 Particle Detectors 3 carsten.niebuhr@desy.de 30 Particle Detectors 3 Examples for Sampling Calorimeters Comparison of various Calorimeters (Electromagnetic) Scintillator Light Guide Light Detector For sampling thickness there are additional sampling fluctuations: σ( E) d X 0 Wavelength Scintillating Fibres Shifter Lead Matrix d Charge sensitive Amplifier Absorber + HV "spaghetti" calorimeter 0.0/ Ε 0.0/Ε Liquid (LAr, LXe, LKr) MWPC Streamer Tubes carsten.niebuhr@desy.de 3 Particle Detectors 3 carsten.niebuhr@desy.de 32 Particle Detectors 3

9 High energy hadrons also develop showers in an absorber Shower development much more complicated than in EM case Components in shower hadronic electromagnetic - mainly due to π o invisible - nuclear excitation - neutrons - neutrinos Typical length scale given by nuclear interaction length λ i : N( x) = N ρ 0 exp ----x λ I Hadron Calorimeters n λ I e + e - e - e + π o π - π - Heavy fragment n Excited nuclei Electromagnetic Component Hadronic Component hadronic showers are much longer and much wider than electromagnetic showers carsten.niebuhr@desy.de 33 Particle Detectors 3 Hadronic Interactions For high energies the hadronic cross section is σ [mb] 200 constant as function of energy 00 independent of hadron type Material dependence of total cross section is given by: σ A σ p A = 5 Characterize hadronic interactions by hadronic 2 A πp interaction length λ I = [cm] πd N A ρ σ A [in table: λ [g/cm 2 I = λ I ρ A 3 ] ] λ I, X 0 (cm) 0 3 λ I interaction lenght 0 2 ~ 0 X 0 radiation length carsten.niebuhr@desy.de 34 Particle Detectors 3 Z Cross section (mb) Cross section (mb) Center of mass energy (GeV) 200 pp π + p pp total elastic π + p total π + p elastic σ T σ T Center of mass energy (GeV) Problem Compensation the fraction of the different components fluctuate significantly the signal response of electromagnetic and hadronic component are in general different i.e. ε em ε had for good performance one needs to compensate for this effect. Two possibilities: hardware compensation - careful choice of absorber & active material and their thickness - increase ε had : Uranium [neutrons and soft γ: fission] or increase neutron eff. w/ hydrogenous comp. - decrease ε em : reduce sensitivity to low-energy γ by using high Z absorber and low Z detector - example: ZEUS calorimeter: Uranium (depleted) / scintillator [3.3/2.6 mm] software compensation - if readout granularity of detector is sufficiently high one can distinguish between electromagnetic and hadronic component and correct by software weighting - example: H calorimeter: liquid argon (LAr) with steel [ cells] due to the large intrinsic fluctuations hadronic calorimeters in general have worse resolution compared to electromagnetic calorimeters typical values: σ( E) % carsten.niebuhr@desy.de 35 Particle Detectors 3 Examples for Calorimeters Experiment Kind Absorber Active material Resolution Type ZEUS em Uranium Scintillator 8% / E sampling ZEUS had Uranium Scintillator 35% / E sampling H em Pb LAr 2% / E sampling H had Steel LAr 50% / E sampling H SpaCal em Pb Scintill. Fibre 7.5% / E sampling NA48 em LKr LKr 3.5% / E homogeneous BaBar em CsI CsI 2.3% / E /4 homogeneous ATLAS em Pb (Cu) LAr 0% / E sampling ATLAS had Iron (Cu) Scintillator 47% / E sampling CMS em PbWO 4 PbWO 4 4% / E homogeneous CMS had Brass Scintillator 5% / E sampling carsten.niebuhr@desy.de 36 Particle Detectors 3

10 LHC Calorimeters under Construction Calorimeter R&D for DESY At Linear Collider expect final states with heavy bosons: W, Z, H CMS ECAL ~75000 Crystals ~75000 Crystals ~20000 channels ATLAS ECAL Have to reconstruct their hadronic decay modes multi jet events Challenge: need jet resolution of 30% / E Expect ~60% of total energy in charged particles (tracker), 20% in photons (ECAL), 0% in neutral hadrons (HCAL) and 0% in neutrinos new concept of particle flow: - reconstruction of individual particles - separation of particles (charged and neutral) Detector requirements: - excellent tracking, in particular in dense jets - excellent granularity in the ECAL - no material in front of ECAL - good granularity in the HCAL - excellent linking between tracker ECAL HCAL - excellent hermeticity Prototype tests ongoing at CERN carsten.niebuhr@desy.de 37 Particle Detectors 3 carsten.niebuhr@desy.de 38 Particle Detectors 3 Summary on Calorimeters High energy particles develop showers: e, γ electromagetic showers, relevant quantities: E c, X 0 shower development: - longitudinal t max loge - transverse Moliere Radius hadrons (π, n, p,...) hadronic showers, characteristic quantity: λ I - shower development much more complicated than in em case Many effects contribute to energy resolution σ dominant dependence from stochastic fluctuations: calorimeters complementary to tracking detectors typical resolutions σ 4 0% - electromagnetic: ; hadronic: σ 35 50% carsten.niebuhr@desy.de 39 Particle Detectors 3